Differential Dry Pipe Valves
Many technicians understand the basics of how a dry pipe valve works, but often the specifics of how differentials work is not known. In this blog we will explore how to determine the trip pressure of a dry pipe valve and how to determine the proper air pressure for these valves, or what is known as the differential. We will discuss both legacy as well as other differential valves.
Understanding Dry Pipe Valves
One must understand the components of a dry pipe valve, as well as how it works, in determine the differential of a valve. Below is a picture of a legacy dry pipe valve and, like every alarm valve, it has three chambers. The lower chamber, which is where the water enters the valve. The intermediate chamber, which is open to atmospheric pressure and where the water for the waterflow alarm comes from, and the upper chamber which is everything above the clapper.
The clapper or upper chamber surface area is larger than the lower chamber to create the differential in supervisory air or nitrogen pressure to water pressure. Most newer valves operate on a differential but add a mechanical mechanism that holds the clapper in the set position, where a legacy valve such as in the picture operates purely on a pneumatic basis.
Understanding Differential in Fire Protection
Let’s explore what a differential is and how it works. The differential is how many pounds of air or nitrogen it takes to hold back a specific amount of water pressure. For example, a Tyco DPV-1 has a 5.5:1 ratio deferential. This means that 1 pound of air will hold back 5.5 pounds of water. In order to determine our normal air pressure, the incoming water pressure must be known.
An incoming water supply of 100-psi, we would divide the water supply pressure by a differential of 5.5. This would give us a trip pressure of 18-psi. When the manufacturer’s instructions are not available or used, NFPA 13 requires 20 psi minimum in excess of the calculated trip pressure. In this case 20-psi required pressure plus the 18-psi trip pressure would provide a normal operating pressure of 38-psi. Table B of Tyco’s data sheet TFP1020 (DPV-1) states that an air supply of between 25 and 33-psi would be needed for 100-psi of water.
Courtesy of Tyco Fire Protection
Clearly there is a difference in supervisory air or nitrogen pressure and if available, the manufacturers’ recommendations should always be followed. This becomes very important when low pressure actuated valves are in use. The Reliable EX dry pipe valve for example has a variable differential based on incoming water supply as outlined in the table below. If a low pressure actuated valve was in use, and the manual calculation was used, trip pressure plus 20-psi, then the supervisory air pressure would almost always be too high, in turn delaying the operation of the dry pipe valve.
Courtesy of Reliable Sprinkler Company
At times when technicians encounter dry pipe systems that are falsely tripping, they will increase the air pressure to keep the valve from tripping until a time can be arranged to troubleshoot the system properly. This is not good practice as it will essentially prohibit the dry pipe valve from operating properly.
An obstruction investigation is a commonly missed requirement of NFPA 25 for dry pipe valves that abnormally trip. NFPA 25 section 14.3.1 (10) states that abnormal tripping of dry pipe valves shall have an obstruction investigation completed.
Technicians that do not normally conduct inspection and testing in accordance with NFPA 25 are not exempt from knowing what the requirements are for conducting obstruction investigations as well as what the requirements are for making repairs, changes, and modifications to fire sprinkler systems. These requirements are outlined at the end of each chapter of NFPA 25, in the summary of component action requirements table.
One of the last things to consider when verifying supervisory air pressure requirements is priming water. This is also something that can be found in the manufacturer’s data sheets. Almost all legacy dry pipe valves will need priming water and usually more water than one would think.
A 6” central model AG/AF will require four gallons of priming water. How many times have we seen only a few water bottles used? Transversely most new dry pipe valves require no priming water, as these valves are required to have high water level protection, in accordance with NFPA 13. This is allowed to be a valve at the level of the clapper and can be manual or automatic. Quarterly, in accordance with NFPA 25, priming water levels are to be verified so that water columns do not form or effect the operation of the dry systems.
In the end, as with all fire protection systems, the key is to know what is required by the standards and follow the recommendations of the manufacturers to ensure to a reasonable degree, that if a fire sprinkler system is needed, it will operate.
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